Which option best describes the correct sequence of electrical conduction in the heart starting from the brain?

• A. Brain to SA node to AV node to Purkinje fibers; ventricles contract
• B. Brain to AV node to SA node to atria to Purkinje fibers
• C. Brain to SA node; impulses to atria; impulses travel to AV node; Purkinje fibers activate ventricles
• D. Brain to SA node to Purkinje fibers to AV node to atria

Answer: (C)

Electrical conduction starts with a natural pacemaker in the heart, the SA node, which sets the pace and can be modulated by the brain through the autonomic nervous system. From the SA node, the impulse spreads across both atria, causing them to contract and push blood into the ventricles. Once the signal reaches the AV node, there is a brief delay that allows the ventricles time to fill. The impulse then travels down the conducting system to the ventricles via the bundle of His and Purkinje fibers, triggering ventricular contraction. This sequence—signals from the brain to the SA node; then to the atria; then to the AV node; and finally through the Purkinje fibers to activate the ventricles—fits the way the heart coordinates atrial followed by ventricular contraction.